1. Field of the Invention
This invention relates to the removal of sulfur dioxide and particulate material from boiler flue gases. In particular, this invention relates to a process and system for dry scrubbing desulfurization of flue gases using soda ash, lime, or similar alkali reagents.
2. Description of the Prior Art
The wet scrubbing of boiler flue gases with alkali slurries to remove sulfur dioxide is well known. Although wet scrubbers are effective and have enjoyed wide application, a drawback associated with their use is the difficulty of waste product disposal. That is, the common by-product of the desulfurization of flue gases by wet scrubbing is sludge which may contain up to 50 weight percent water. Sludge requires comparatively expensive disposal means, such as on site settling ponds or costly transportation to remote disposal sites, as well as some stabilization expenses.
In order to overcome this disadvantage of wet scrubbing, it has been proposed to utilize a dry product system in which an atomized solution or slurry of alkali reactant is mixed with flue gas in a spray dryer so that the sulfur dioxide contained in the flue gas is absorbed on the surface of fine drops in the atomized spray and the alkali material reacts with the sulfur dioxide to form sulfite and sulfate compounds. A dry powder mixture of these compounds, with residual unreacted alkali, is formed as a result of the thermal energy of the flue gas. The flue gas, containing a particulate mixture of the reaction products, unreacted alkali and any flyash originally present, exits the spray dryer and is transported to dust removal equipment such as a fabric filter or electrostatic precipitator. The cleaned gas is then exhausted and the particulates are removed from the dust collector hoppers in a dry form for disposal.
It is stated that advantages of dry scrubbers over wet scrubbers include simplicity of operation, lower capital costs for systems, smaller energy demands of operation as well as easier transportation and handling of dry wastes for end product disposal. Environmental requirements may also be more readily met by such systems.
In the articles "Two-State Dry Scrubbers Come of Age Interest Booms, Orders Start Coming In", Electric Light and Power, September, 1978, pp. 49-50, and "Tests of a Two-Stage Combined Dry Scrubber/SO.sub.2 Absorber Using Sodium or Calcium", Combustion, November 1978, pp. 30-43, a dry scrubbing system is described. In that system, boiler flue gas enters a spray dryer where a mist with a dilute reagent solution or slurry is sprayed through the gas. Quick chemical reaction is said to remove a substantial portion of the sulfur dioxide from the flue gas while the heat in the gas evaporates the water and dries the solution to form a dry powder. Flue gas containing the dry powder and flyash leaves the spray dryer and enters either a precipitator or fabric filter dry collector where, it is said, additional reaction of the sulfur dioxide takes place as the flyash and powder are removed from the gas in the collector. The clean gas is then exhausted to a stack while flyash and sulfur dioxide-containing powder particulates are removed from the collector hoppers in dry form for disposal. Among the dry solids recovered are the reagent materials, such as lime, which may be recycled as absorbent feed to the slurry feed tank to be mixed with make-up lime slurry before being atomized in the spray dryer for further use.
The prior art dry scrubbing systems are currently essentially still in the pilot testing stage. In Fabric Filter Newsletter, Nov. 10, 1978; No. 37, pp. 3-4, it is stated that 90 percent sulfur dioxide collection may be achieved using either soda ash or lime as a reagent. With soda ash, this level of sulfur dioxide removal is said to be obtained with a stoichiometric ratio of 1.0 to 1.2 while using lime as an absorbent required to stoichiometry of between 2.3 and 3.0 for the same removal efficiency. With recirculation of dried and partially reacted product and flyash from the spray dryer and filter bag house to the feed tank for mixing with make-up lime slurry before being again atomized, lime utilization is said to be increased with a 90 percent removal efficiency allegedly being obtained with a stoichiometry in the range of 1.3 to 1.7.
Although the prior art dry scrubbing systems present the inherent advantages that dry scrubbing provides over wet scrubbing, they nevertheless incur high reagent costs due to the high stoichiometric ratios of reagent to sulfur dioxide required to obtain the desired removal efficiency. For example, lime removal efficiencies of 90 percent are attained only at undesirably low utilization efficiences. This is a particular drawback where sulfur contents in coal are high, such as above about 1.4 percent. Also, where filter bag house, or other dust collector, discharge powder is recycled to the feed tank, the amount of recycle is severely limited by the amount of water which can be used, which is determined by the inlet and outlet temperatures of the spray dryer, and the upper limit of slurry concentration which the spray dryer atomizer can accept.
Thus, there exists a need for dry scrubbing processes and systems which possess increased efficiencies and result in reduced reactant costs and, concurrently, decreased waste amounts and reduced disposal costs.